1. Field of the Invention
The present invention relates to a solid-state laser having a nonlinear optical crystal which is incorporated into a resonator for converting the wavelength of a laser beam, and more particularly relates to a solid-state laser designed in such a way that a wavelength-converted wave having a high output is obtained by adjusting the length of the resonator to an optimum length. Moreover, the present invention relates to a method for manufacturing the previously mentioned solid-state laser.
2. Description of the Prior Art
As disclosed in, for example, U.S. Pat. No. 4,656,635, a solid-state laser is publicly known in which a solid-state laser, doped with rare earth elements such as Neodymium, is pumped with a semiconductor laser or the like.
In this type of solid-state laser, in order to obtain a laser beam having a shorter wavelength, it is widely practiced that the wavelength of a solid-state laser is converted to a second harmonic wave by disposing crystal made of a nonlinear optical material in a resonator of the laser. Further, in the field of such a solid-state laser, various proposals have been put forward in which an etalon for selecting an oscillation wavelength is disposed in a resonator to select a single longitudinal mode.
When the temperature of the resonator disposed in such a solid-state laser is changed, optical members of the resonator and mechanical members, such as a copper block for fixing the optical members, undergo thermal expansion. This causes the refractive indices of the optical members to vary, which in turn results in variations in the length of the resonator. If the temperature of the resonator is changed in the manner as mentioned above when the etalon is disposed within the resonator, a resonator mode that depends on the length of the resonator might, or might not, be matched with an etalon mode. This causes an output of the solid-state laser to be varied.
A curve "a" shown in FIG. 1 represents the outline of the relationship between the temperature T of a resonator and an output P.sup..omega. of a solid-state laser in the above mentioned situation. As can be seen from the drawing, the maximum value of the output P.sup..omega. is obtained when the resonator temperature T=T.sub.1, at which a resonator mode is matched with an etalon mode. The output gradually drops as the resonator temperature T shifts away from T.sub.1, and the output becomes minimum when the temperature T=T.sub.2, at which the resonator mode is switched to another mode.
When the wavelength of a solid-state laser is converted by the use of a nonlinear optical crystal disposed in the resonator, an output p.sup.2.omega. of a wavelength-converted wave (for example, a second harmonic wave) varies in accordance with the temperatures of the crystal, i.e. the temperature T of the resonator, as represented by a curve "b" shown in FIG. 1. Specifically, the efficiency of conversion of a wavelength becomes maximum when the resonator temperature T=T.sub.3. However, if the resonator temperature T.sub.2, at which the resonator mode is switched to another mode, is matched with or close to the resonator temperature T.sub.3, the output p.sup.2.omega. of the second harmonic wave will not be increased so much even when a high efficiency of wavelength conversion is obtained by setting the resonator temperature T to T.sub.3, because an output of the solid-state laser, which serves as a fundamental wave, is itself small.
Further, in this case, if the resonator temperature T is set to T.sub.3, longitudinal modes at low temperatures of the resonator and longitudinal modes at high temperatures of the resonator simultaneously exist, and this induces mode competition. For this reason, noise develops in a wavelength-converted wave.
To solve this problem, a proposal has conventionally been put forward in which resonator mirrors are moved in the direction of the optical axis using piezoelectric elements so as to vary a resonator mode, so that the relationship between the resonator temperature T and the output P.sup..omega. of the solid-state laser is changed like a curve "c" shown in FIG. 1. In other words, with such a construction, it is possible to sufficiently increase the output p.sup.2.omega. of the second harmonic wave by making the resonator temperature, at which the output P.sup..omega. of the solid-state laser becomes maximum, coincident with or close to the resonator temperature, at which the maximum wavelength conversion efficiency is obtained.
The previously mentioned construction, in which the resonator mirrors are moved in the direction of the optical axis using piezoelectric elements in the manner as mentioned above, provides poor long-term reliability. Therefore, the long-term reliability of the solid-state laser is impaired.